Iron-sulfur proteins are present in all living matter and represent one of the most distributed proteins in nature. They constitute a class of important biological electron transfer agents, but are increasingly found to have more biological functions. Their functionality is critically dependent on the electronic structure of their active sites and the influences imposed by the protein environment. The active sites most frequently consist of a one-Fe center, Fe(2)S(2), Fe(3)S(4), and Fe(4)S(4) clusters, usually coordinated by sulfur from the cysteine side chains of the surrounding proteins. Nature has evolved a diverse range of functionality with just the few Fe-S clusters through fine-tuning of the protein environment. It is thus crucial to understand the properties of the Fe-S clusters and modifications by their surroundings in order to understand the properties and functionalities of the iron-sulfur proteins. However, Fe-S clusters are extremely complicated electronic systems. This proposal seeks to experimentally probe, in the gas phase, the electronic structure and properties of the Fe-S clusters in their bare forms, in analog complexes, and in proteins. The gas phase investigations are free from the perturbation of solvents or crystal field, yields the intrinsic properties of the Fe-S clusters, and allows the influences of the protein environments to be probed directly. The experimental approaches in this proposal are unconventional, taking advantages of recent developments in biological mass spectrometry and gas-phase spectroscopic techniques. An electrospray ion source will be used to transport Fe-S clusters and proteins into the gas phase. Negatively charged ions of the Fe-S clusters and proteins will be interrogated by photodetachment photo-electron spectroscopy, which yields direct information about their electronic structures and intramolecular electrostatic interactions. Because photodetachment is analogous to an oxidation process, the proposed experiments will also yield energetic information relevant to redox reactions of Fe-S clusters. It is expected that the new experimental results from the proposed work will stimulate further theoretical advancements.